Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 211729
SWODY2
SPC AC 211728

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1228 PM CDT Sat Apr 21 2018

Valid 221200Z - 231200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
THE LOWER MISSISSIPPI VALLEY AND CENTRAL/EASTERN GULF COAST...

...SUMMARY...
Isolated severe thunderstorms are possible across portions of the
lower Mississippi Valley east to the Florida Panhandle on Sunday.

...Synopsis...
An upper-level low centered near the OK/AR border at 12Z Sunday will
move east during the day as a 70 kt mid-level speed max rotates
through the base of the trough.  Surface low pressure, initially
over northern LA, will weaken as it lifts northward throughout the
day and becomes vertically aligned with the upper low.  A cold front
will pivot east across the lower MS Valley and central Gulf coast
region through Sunday night, and a warm front will extend across
southern portions of the central Gulf Coast states.

...Lower MS  Valley/central Gulf Coast...
Considered introducing a small Slight Risk centered over portions of
southern MS for Sunday, however with uncertainties regarding the
degree of instability/related impacts from early day precipitation
will defer to later outlooks for any possible adjustments.

Showers and thunderstorms will likely be ongoing at 12Z Sunday
associated with height falls/large-scale forcing for ascent coupled
with weak low-level warm advection.  In the wake of morning
precipitation, filtered diurnal heating of a moist air mass (surface
dew points lower/mid 60s) should contribute to at least weak
surface-based instability within the warm sector, and strengthening
wind fields with the approaching speed max will result in deep-layer
shear of 40-50 kts.  Additional thunderstorm development is expected
during the afternoon near the cold front, with any initially
semi-discrete storms evolving into a linear structure given the
frontal forcing.  Some guidance indicates a secondary surface low
developing along the cold front over southern MS Sunday afternoon,
possibly near the intersection with the effective warm front.  This
would somewhat enhance low-level SRH and the potential for low-level
storm rotation.  High resolution guidance does develop isolated
storms over southern MS during the afternoon, and at least some
tornado risk would exist in this scenario.  As storms become more
linear with time, damaging winds would become the primary threat
along with the potential for embedded QLCS circulations.  Given
uncertainty regarding the magnitude of surface-based instability and
the latitudinal position of the warm front, will maintain
categorical Marginal Risk with this outlook.

Farther north, the potential exists for isolated storms to develop
over portions of eastern AR near the upper low.  Cooler mid-level
temperatures will contribute to modest instability and some risk for
hail with the strongest updrafts, and a brief tornado cannot be
ruled out.  The Marginal Risk was extended northwest based on this
reasoning.

..Bunting.. 04/21/2018

$$



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